Subtleties – Part 2 (Nested Tables and Varrays)

In Part 1 we saw that the SQL function COLLECT with the DISTINCT option is not natively supported in PL/SQL.
One suggested workaround was to apply the SET function on the result of the “simple” COLLECT function (without the DISTINCT option).
This works fine, in both SQL and PL/SQL, as long as the collection type that we use is Nested Table.

create type integer_ntt as table of integer
/

select person_id,set(cast(collect(project_id) as integer_ntt)) project_id_list
from project_assignments
group by person_id
order by person_id;

PERSON_ID PROJECT_ID_LIST
---------- -----------------------------------------
       101 INTEGER_NTT(1, 3, 2)
       102 INTEGER_NTT(2)
       103 INTEGER_NTT(3)

But what if the collection type we use is Varray and not Nested Table?

create type integer_vt as varray(100) of integer
/

select person_id,set(cast(collect(project_id) as integer_vt)) project_id_list 
from project_assignments 
group by person_id
order by person_id;

select person_id,set(cast(collect(project_id) as integer_vt)) project_id_list
                     *
ERROR at line 1:
ORA-00932: inconsistent datatypes: expected UDT got DEMO.INTEGER_VT

So not every operation we can do with one collection type (Nested Table) is supported with other collection type (Varray).
SET is one example for that, and actually it makes sense. The elements within a set are unordered by definition, while a Varray is an ordered list of elements.
Another example for a function that works for nested tables but not for varrays is the CARDINALITY collection function:

select cardinality(integer_ntt(7,8,9)) from dual;

CARDINALITY(INTEGER_NTT(7,8,9))
-------------------------------
                              3

select cardinality(integer_vt(7,8,9)) from dual;

select cardinality(integer_vt(7,8,9)) from dual
                   *
ERROR at line 1:
ORA-00932: inconsistent datatypes: expected UDT got DEMO.INTEGER_VT